This invention relates to sterile culture media dishes.
It is old and well known to utilize culture media in a container for conducting growth and biochemical reactions, wherein the sample of the specimen to be tested is applied to the surface of the medium in the container. Such devices are commonly known as Petri dishes or plates.
Commonly used Petri dishes in the form of a dish having a bottom wall and a peripheral side wall are filled with the molten culture medium and rapidly cooled to solidify or gel. A cover is loosely applied. In order to prevent surface phenomenon problems such as contamination and condensation of moisture, it has been essential to refrigerate such Petri dishes until they are to be used. Contamination occurs because the cover is loosely applied allowing entrance and exchange of external non-sterile atmospheres.
Syneresis which is the separation of liquid from a gel occurs because of evaporation and contraction of the culture medium and the inability of the expelled liquid to rehydrate the solidified gel. When there is a temperature differential between the inside of the dish and the environment, the effect of this evaporation on the inside cover results in heavy condensation and necessitates that a technician must first dry the interior of the plate before using it. Dehydration occurs more rapidly with warmer temperatures and dry conditions in the environment.
U.S. Pat. No. 4,262,091 shows a method for providing Petri dishes of a culture medium in an oxygen-free manner. The empty culture dishes require prior anaerobic storage, and the dishes are stored, filled and packaged under anaerobic conditions. This process is extremely cumbersome and expensive especially for high volume production of culture dishes.
In an effort to reduce or prevent problems with sterility and dehydration, it has been common to provide a, plurality of such Petri dishes in a sterile plastic sealed bag. However, it has still been essential to refrigerate the package for most types of culture media to retard syneresis, contamination and dehydration. Refrigeration has also been necessary for culture media which are subject to oxidation such as those that contain blood, vitamins or antibiotics. Further, refrigeration may be required for extended storage greater than a week. Currently, the shelf life of such culture dishes is very limited, and is on the order of three or four months. Accordingly, it has been necessary to date the package. As a result, if the package is not used within the time indicated it must be destroyed. In addition, the manner in which the dishes are packaged does not protect them from breakage.
Therefore, there is a need for a system which is more convenient and does not require that dishes be placed in anaerobic storage before being filled with medium; in which the dishes may be filled with a medium in a conventional manner under ambient conditions; which provides a package system that is designed to protect the dishes from breakage; which allows ease of transportation of the dishes; and which permits visual inspection prior to use.
Among the objectives of the present invention are to provide a culture media package which has a long shelf life; which minimizes the problems of syneresis, desiccation, and contamination; which does not require refrigeration; which can be readily shipped; which is not fragile; which can be utilized in a conventional manner as in the well known art of using Petri dishes; which is low in cost; which can be produced relatively rapidly; which is transparent, allowing for visual inspection prior to use; and which is pleasing in appearance and which will withstand shipping and handling without special precautions.
In accordance with the invention, a culture package system comprises a package such as a container containing one or more conventional culture media dishes containing solidified culture media. Each dish includes a bottom wall and a peripheral side wall and a removable cover having a bottom wall and a peripheral side side wall which is loosely telescoped over the dish. A plurality of the dishes are preferably provided in inverted stacked relation in a package, such as a container. A cover is hermetically sealed to close the container. An oxygen absorber is placed in the package to facilitate maintaining the atmosphere at a low oxygen level. The interior of the package has a gaseous atmosphere having minimal oxygen content, preferably less than 1% oxygen. Preferably a moisture absorber is also sealed in the package. In addition, a color indicator is preferably sealed within the package to indicate by change of color when the oxygen level within the container exceeds a predetermined value.
The container is made of plastic material which is impermeable to oxygen and moisture and is preferably in the form of a container which has a base wall and a peripheral wall. The container preferably includes means for preventing movement of the dishes laterally. The peripheral wall preferably has a series of ribs which are concave inward and a series of ribs which are concave outward and disposed around the circumference of the peripheral wall. The inwardly concave ribs are constructed and arranged to contact the exterior surface of the dishes and to engage and cushion the dishes so as to prevent breakage. The outwardly concave ribs facilitate placement of the absorbers into the container and removal of the dishes from the container. Preferably a partial vacuum exists within the container causing the cover to flex into engagement with the stack of dishes and hold the dishes firmly within the container.
In accordance with the invention there is also provided a method of making the sterile package system including a package preferably in the form of a container containing a plurality of culture media dishes, each dish including a bottom wall and a peripheral side wall and a removable cover having a bottom wall and a peripheral side wall telescoped over the dish. The method comprises the steps of:
1) filling the dishes successively with a culture medium,
2) successively drying the head space above the media, by sterile, low humidity air, without the need to cool the dishes,
3) applying a cover successively to each dish,
4) thermoforming successive containers of plastic material comprising a base wall and a peripheral wall,
5) inserting a plurality of covered dishes in stacked inverted relation in each container,
6) inserting an oxygen absorber packet and optionally a moisture absorber packet into the container,
7) preferably flushing the containers with an oxygen free gas, and
8) sealing a plastic top on each container.
Preferably, the package system is made by thermoforming successive containers from a strip of thermoplastic material to provide a strip of interconnected containers. The cover is also preferably made from a continuous strip of plastic material. The containers are successively filled with a plurality of the conventional culture media dishes and the oxygen absorber packet, the optional moisture absorber packet and the color indicator packet are inserted. The oxygen and moisture absorber packets are preferably placed in the container after the culture medium dishes are placed into the package. A stream of oxygen free gas is then used to flush the package, and the containers are successively sealed by the cover. The packages are then successively severed from the strip.
In the method of making a package system to provide stability and increased shelf life, the culture dishes may be filled with medium in an aerobic atmosphere, that is, without the necessity for providing an anaerobic atmosphere. In addition, the package can be made without the necessity of cooling by refrigeration. Preferably, the containers are flushed with a nitrogen gas, thereby creating anaerobic conditions in the container. Advantageously, the use of nitrogen gas is very cost effective and therefore no catalysts or gases containing hydrogen are required in the manufacturing process. The oxygen absorber, preferably in the form of a packet, maintains a low oxygen environment within the container.
The atmosphere in the package system is preferably less than ambient atmospheric pressure to secure the plates within the package. When the pressure is less than ambient, the flexible top is flexed inwardly such that it lies tightly against the stack of plates, preventing them from moving about within the package. This helps protect the plates from breakage.